1. Field of the Invention
The present invention relates to an electric power steering system that prevents an assist current from excessively increasing and detrimentally affecting the durability of a gear box and/or the motor of a steering actuator.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2002-234456 (JP '456) discloses an electric power steering system having a steering device which includes a rack-and-pinion type steering gear box, wherein a steering torque is input from a steering actuator to the steering device to assist a steering operation conducted by a driver. In the steering device disclosed by JP '456, overheating of a motor of the steering actuator or a control system due to an overload is prevented by providing an upper limit value on an indicator current supplied to the motor of the steering actuator.
Limiting of the upper value of the indicator current supplied to the motor is initiated when a continuous service time of the motor exceeds a predetermined amount. The upper limit value is first gradually decreased from a predetermined maximum value to a minimum value at a predetermined gradually decreasing rate. The upper limit value of the supplied indicator current is then increased to the maximum value at a predetermined gradually increasing rate. The conditions for limiting the upper limit value of the supplied indicator current are less severe when a temperature sensor detects a decrease in the atmospheric temperature.
In the steering device disclosed by JP '456, when a rack bar of the steering gear box cannot be further moved after reaching a lateral stroke end, the steering torque detected by the steering torque sensor is steeply increased. Therefore, an assist current supplied to the motor of the steering actuator is also steeply increased. In order to accommodate such a steep increase in the assist current, the assist-current limiting technique disclosed by JP '456 is insufficient. Therefore, there is a need in the industry for a more reliable assist-current limiting technique.